Light emitting elements which use a nitride semiconductor and have a wavelength band of 370 nm-550 nm have been made commercially available. In these light emitting elements, InxGa1-xN (0<x<1) is used as a light emitting material. The light emission wavelength changes when a compositional ratio of In in InxGa1-xN is changed. Specifically, the light emission wavelength is increased as x is increased. When the compositional ratio x of In is changed, the light emission efficiency also changes in addition to the light emission wavelength. More specifically, when the compositional ratio x of In is increased too much, because (1) a difference in lattice constants between InGaN and the layers sandwiching InGaN, that is, between InGaN and GaN or between InGaN and AlGaN, becomes large and (2) a crystal growth temperature must be reduced in order to grow crystal of InGaN having a high composition of In, the crystal quality of InGaN is degraded and the light emission efficiency is reduced when the wavelength exceeds 530 nm. In general, in a wavelength range of 400 nm-530 nm, the light emission efficiency is increased, but the light emission efficiency is reduced when the wavelength is 400 nm or smaller.
A reason why the light emission efficiency at a shorter wavelength side of 400 nm or smaller is reduced may be considered to be due to dislocations present in the crystal. An efficiency of a light emitting element (LED or the like) having a wavelength of 400 nm-530 nm having an arbitrary compositional ratio of In is high regardless of a dislocation density because of a fluctuation of In composition within the InGaN layer. More specifically, when there is a compositional fluctuation of In, light is emitted at a local region in which the In composition is large and the injected carriers are captured in this local region. As a consequence, the carriers do not reach the dislocations and the efficiency is not reduced. As described above, in order to shorten the light emission wavelength, the compositional ratio x of In must be reduced, which inevitably results in reduction in the fluctuation of In composition. When the fluctuation in composition is small, the carriers are not sufficiently captured. As a consequence, the carriers reach the dislocations and the light emission efficiency is reduced.
As described, when the light emission wavelength is 400 nm or smaller, the light emission efficiency significantly depends on the dislocation density and is reduced due to the presence of dislocations.
In order to prevent reduction of light emission efficiency in a wavelength band of 400 nm or smaller, the dislocation density must be maintained at a low level. The dislocation density is reduced in the related art, for example, through an ELO (Epitaxial Lateral Overgrowth) method and a method in which a light emitting layer is grown on a sapphire substrate or the like onto which a groove is formed. However, because these methods require photolithography or the like, there had been a problem in that the manufacturing requires labor and the cost of the light emitting element is increased.